Continuous glucose monitoring devices use the technique of reverse iontophoresis to extract the glucose analyte from the interstitial fluid of a patient for the purpose of measuring changes in glucose levels. Such devices take the form of a patch held in intimate contact with the skin of a patient and comprise a sensing chamber containing electrochemical sensors which are arranged to form part of an electrical circuit. The chamber is filled with a fluid medium or a gel through which the analyte can diffuse or be transported from the skin to the sensors. Either the medium or a surface of the sensors is impregnated with an enzyme that reacts with the analyte to convert it into a different form (gluconic acid in the case of a glucose analyte) and to produce electrons in the process. The electrons can flow around the circuit to create a current, the magnitude of which corresponds to the concentration of the analyte in the medium surrounding the sensor. However, the magnitude of the current is typically only tens or hundreds of nanoamps so it is a challenge to measure it accurately and to distinguish the current signal from background electrical noise. In addition, the sensors may not operate in a reliable, linear manner at low concentrations of the analyte in the sample.
Known devices are operated to carry out an initial period of reverse iontophoresis for analyte extraction, followed by a period of sensing which is often 2 to 3 times longer than the extraction period, so as to ensure all the analyte that has been extracted from the subject during the preceding period has time to react with the sensors. One glucose monitoring device that was commercially available under the trade mark Glucowatch operated with an iontophoresis period of 3 minutes followed by a sensing period of 4 to 5 minutes. The cycle is repeated at intervals, each time a new reading of the subject's glucose level is required, and on each occasion the sensing period is long enough to deplete substantially all of the extracted glucose in the sample.